This invention relates to ultrasonic imaging of perfusion of an anatomical region and, more particularly, to a method for enabling imaging of a contrast agent with reduced tissue clutter.
Current ultrasonic imaging systems make use of contrast agents in circulation to enhance ultrasound returns. Contrast agents are substances which strongly interact with ultrasound waves and return echoes which may be clearly distinguished from those returned by blood and tissue. Microbubbles are currently employed as a contrast agent and provide a non-linear behavior in certain acoustic fields. Such behavior is readily detectable by use of known algorithms. Microbubble contrast agents are useful for imaging of the body""s vascular system and are injectable through the veins and arteries. They are subsequently filtered from the blood stream by the lungs, kidneys and liver.
Microbubble contrast agents generally comprise coated gas bubbles that are stable in the body for a significant period of time. The coating shells serve to protect the gas from diffusion into the blood stream. At moderately high ultrasound pressure amplitudes, the shells of the microbubbles can be caused to rupture, freeing the internal gas and substantially eliminating the detectability thereof by incident ultrasound waves.
U.S. Pat. No. 5,410,516 to Uhlendorf et al. describes an ultrasound system that produces images from echo returns from a microbubble contrast agent. The echo signals that are used for imaging are those that exhibit harmonic and subharmonic relationships to the fundamental transmission frequency.
U.S. Pat. No. 5,833,613 to Averkiou et al. discloses an ultrasound method for imaging of contrast agents. In one embodiment, a rate of re-perfusion of an anatomical region is accomplished by initially destroying the contrast agent within the region, and then subsequently imaging the region to determine the rate of re-insertion of the contrast agent. The Averkiou et al. method of indicating the rate of re-perfusion utilizes plotted curves that indicate echo returns from interrogating ultrasound beams. Initially, Averkiou et al. transmit high energy ultrasound pulses to destroy the microbubbles in the region to be imaged. A short time later, lower energy, imaging, ultrasound pulses are transmitted again, the echoes received and imaged to measure the degree of microbubble re-infusion by, for example, counting or integrating the pixels in the area which show re-infused microbubbles. The measure of the number of re-infused microbubbles in the region is plotted in curve format. Non-destructive pulses can thereafter be repetitively transmitted and echoes received and plotted as a sequence of points to indicate the rate of re-perfusion.
U.S. Pat. No. 5,879,303 to Averkiou et al. discloses still another ultrasound method for imaging of contrast agents. In this patent, a programmable digital filter is used to pass harmonic echo components to the exclusion of fundamental frequency components of the transmitted signal. The system uses decorrelated replicas of the harmonic signal that are then combined and used for imaging. To produce an image in the presence of depth dependent attenuation of high frequency echo signals, both fundamental and harmonic signals are processed and used to produce an image blended from components of both the fundamental and harmonic signals.
U.S. Pat. No. 5,577,505 to Brock-Fisher et al discloses an ultrasound method for imaging of contrast agents that achieves increased sensitivity to non-linear responses, particularly second harmonic responses under multiple excitation levels. In particular, the responses gathered from multiple echoes are gain corrected in an amount corresponding to the difference in excitation levels and are then subtracted. The subtraction removes most of the linear tissue response and what remains is the non-linear response from contrast agent.
It is known that the contrast ratio of ultrasound images is limited by a second harmonic response that results from interaction between tissue and incident ultrasound acoustic energy. Such harmonic response is caused by non-linear propagation effects which give rise to second harmonic energy in the transmitted acoustic signal. Further, when contrast agent is destroyed, its acoustic response ceases to be constrained by frequency regions around the transmit frequency or harmonics thereof. More specifically, when a microbubble contrast agent is destroyed, its acoustic response becomes broadband and exhibits energy in a broad spectrum of frequencies.
There is a need for improved ultrasound images from contrast agent. Further, harmonic tissue response in such images should be reduced to enhance the contrast of the contrast agent echo returns so as to improve images that are derived therefrom.
The method of the invention controls an ultrasound system to image a microbubble contrast agent in a region of fluid flow and/or a region of tissue that is perfused by blood or other fluid. A transducer transmits ultrasound acoustic signals having a fundamental frequency f and receives echoes resulting from interaction of the ultrasound signals with both tissue and the microbubble contrast agent. A transmitter controls the transducer to transmit ultrasound signals at a sufficient power level to destroy the microbubble contrast agent. A receiver processes the echoes and selectively extracts signal components that exhibit a noninteger multiple of the fundamental frequency f of the transmitted acoustic signal. The system then produces images, principally from signal components including those that exhibit the noninteger multiple of the fundamental frequency f.